History: What is the definition of an equal-loudness contour?

What is the definition of an equal-loudness contour?

• A graph that shows how our hearing varies over frequency and amplitude.
  Excellent!
• Fletcher-Munson curves.
  This is a type of equal-loudness contour, not the definition.
• We perceive some frequencies as louder than others.
  This is the principle, but not the actual object.
• A graph that shows how our hearing varies over phase and amplitude.
  The graph does not involve phase.